During the handling of a photographic material, such as coating, drying, finishing, winding, rewinding, printing, projecting, and so on, the material surfaces are often harmed by contact friction with other apparatus and between its front and back faces. For example, scratches or abrasions can be brought about on the emulsion and back sides of a photographic material. These scratches or abrasion marks are visible during printing or projecting processes. This causes serious problems in the practical use of the films. Moreover, when the contact friction is high, the photographic materials do not transport smoothly during the manufacturing process or in various exposure, processing, and projection machines. These transport problems may result in product waste. In recent years, the conditions under which the photographic materials are manufactured or utilized have become more severe, because their applications have been extended (for example, in an atmosphere of high humidity and high temperature) or because the methods for their preparation have been advanced (for example, high speed curtain coatings, high speed finishing and cutting, and fast processing). Under these conditions, the photographic materials are more easily scratched.
To lower the contact friction and improve the resistance to damage to surfaces, a lubricant or slipping agent is often used. Examples of the lubricants used for those purposes include silicone fluids as described in U.S. Pat. No. 3,489,567, and wax esters of high fatty acids or high fatty alcohols in U.S. Pat. No. 3,121,060. Problems are encountered in the use of these lubricants. For example, waxes such as Carnauba wax have been used to form the backing lubricant layer. However, they need to be coated from solvents such as propylene dichloride, which is on the EPA P/U highly hazardous list. Furthermore, waxes in most cases have to be applied as a separate layer, which requires an additional coating station and therefore increases product cost.
U.S. Pat. No. 4,766,059 describes a method of making solid spherical beads having a mean size ranging form 0.5 to about 20 .mu.m. The polymer beads contain a polymeric resinous material and a water insoluble wax. However, the process of making such solid beads involves the use of water miscible or immiscible low boiling solvent to dissolve both polymeric materials and wax, and subsequently removal of the solvent or solvent mixture by evaporation. This requires large processing equipment and lengthy processing time, which increases the expenses of making these beads.
In other cases, it is also possible to make lubricant impregnated polymer particles by emulsion polymerization. In such a process, lubricant is first dissolved in an ethylenically unsaturated monomer, and the resultant mixture is added to an aqueous phase that contains surfactants above the critical micelle concentration and a water-soluble initiator. The mechanism of the polymerization process has been subject of much research and is generally agreed to include diffusion of monomer (in this case, both monomer and lubricant) from the monomer droplet (or monomer lubricant droplet) formed by agitation to the growing polymer particles where the actual polymerization proceeds. Thus, such a process is not very stable and lubricant can form oily-like scum in the reaction medium causing latex particles to coagulate.
Therefore, a foremost objective of the present invention is an improved method of making uniformly sized polymer particles for use in imaging elements. The polymer particles have a mean size of less than 500 nm and are impregnated with a water-insoluble lubricant. The polymer particles can be applied from a low hazard organic solvent or solvent mixture and are used to form surface protective layers for imaging element that provide surface slip properties and resistance to physical and mechanical scratch and abrasion.